Known in the art is an information recording disc of an air-sandwich structure in which two disc substrates each having a recording medium-containing layers are arranged so that said layers face to each other with a spaced therebetween by means of outer and inner peripheral spacers, said substrates and spacers being welded together by an ultrasonic welding technique (Japense Patent Laid-open Publication No. 60-103,537).
As materials of the disc substrates of the above-mentioned information recording disc use has been made of polymethyl methacrylate, polycarbonate, polystyrene, rigid polyvinyl chloride, epoxy resins and glass. In place of such materials Japanese Patent application No. 62-15,218 proposes copolymers of ethylene with at least one cycloolefin which copolymers are more suitable for being processed by the ultrasonic welding technique.
The proposed copolymers are excellent in heat resistance, low moisture absorption, chemical resistance, precision moldability, and adhesion to the recording medium-containing layer. In order to successfully carry out the ultrasonic welding it is necessary to improve the weldability between the substrates and spacers and to reduce residual strain due to welding thereby avoiding any birefringence. For this reason spacers made of the same kind of resin have been used. Since the disc substrates are required to be transparent, those made of a transparent resin with no inorganic filters have been use, and in turn spacers made of the same transparent resin with no inorganic fillers have been used.
However, there has been a problem in that when disc substrates, and outer and inner peripheral spacers made of the transparent copolymer with no inorganic fillers added are welded by the ultrasonic welding technique to produce an optical disc, a relatively long period for time of ultrasonic vibration is required and the optical disc so produced frequently exhibits a warp of an intolerably large angle.